Adds mattermost app.

mattermost/versions/10/README.md

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+# Mattermost
+
+Mattermost is an open-source team messaging platform, a self-hosted alternative to Slack.
+
+## Dependencies
+
+- **PostgreSQL** - Database for storing messages and team data
+
+## Configuration
+
+Key settings in `config.yaml`:
+
+- **domain** - Where Mattermost will be accessible
+- **storage** - Persistent volume size (default: `2Gi`)
+- **db.name** - Database name (default: `mattermost`)
+
+## First-Time Setup
+
+1. Add and deploy the app:
+   ```bash
+   wild app add mattermost
+   wild app deploy mattermost
+   ```
+
+2. Visit the app URL — on first load you will be prompted to create an administrator account (email + password).
+
+3. After creating the admin account, complete the onboarding wizard to set up your first team.
+
+## Notes
+
+- The setup wizard only runs on first launch
+- Email invitations require SMTP to be configured in **System Console → Environment → SMTP**
+- Push notifications for mobile apps require a push proxy (or Mattermost Cloud push notification service)